The present invention relates to an indicating system for use in indicating the operating status of various electronic equipment.
In audio equipment and the like, indicating lamps are used, not only for the indication of the switching-on of a power supply, but also for indication of tuning accuracy during the reception of a radio broadcast, indication during the play of a record, and the indication of various modes of use in a tape recorder. In this regard, each of the circuit arrangements constituting the electronic equipment as mentioned above is, for the most part, fabricated in the form of a semiconductor integrated circuit. It is therefore desirable that the indicating driver circuit is also formed as a semiconductor integrated circuit and constructed on an identical chip along with an amplifier and other circuit elements of the equipment.
The indicating driver circuit illustrated in FIG. 1 was developed by me prior to the present invention, and it is to this type of circuit that the advantages of the present invention are particularly applicable, as will now be described. When the base of an NPN transistor Q.sub.1 is supplied with a lighting signal V.sub.in, this transistor Q.sub.1 turns to the "on" state. Then, a voltage drop develops across a load resistor R.sub.1, so that a PNP transistor Q.sub.2 is supplied with a base voltage V.sub.BE2 and is turned to the "on" state. As a result, current flows along a path which consists of a +V.sub.CC voltage source, the transistor Q.sub.2, a bias resistor R.sub.2, an output terminal T.sub.1, a resistor R.sub.3, a light emitting diode 1 and ground. In consequence, a voltage drop develops across the bias resistor R.sub.2, and it supplies a base voltage V.sub.BE3 to a transistor Q.sub.3. The transistor Q.sub.3 is turned to the "on" state, and a driving current flows along a path consisting of the +V.sub.CC voltage source, transistor Q.sub.3, output terminal T.sub.1, resistor R.sub.3, light emitting diode 1 and ground, so that the light emitting diode 1 is lit up. The transistor Q.sub.2 and the bias resistor R.sub.2 maintains the transistor Q.sub.3 in the conductive state so long as the lighting signal V.sub.in is received.
With the circuit arrangement as described above, however, one external connection terminal, namely, output terminal T.sub.1 is required for lighting up each light emitting diode 1. Accordingly, in a case where different modes of operation of the equipment are selected by turning a knob 2 counterclockwise and clockwise between successive switch positions as illustrated in FIG. 2, two indicating driver circuits must be provided in order to light up the two light emitting diodes 1 and 1'. This signifies the necessity of providing two external connection terminals for the purpose of lighting up the two indicating lamps, and forms a factor of increased cost in the case of fabricating the semiconductor integrated circuit which includes these two indicating driver circuits. Moreover, with the prior art circuit arrangement, even when two separate indicating driver circuits are provided, they can indicate only the two modes, in one of which the first indicating lamp is lit up and in the other the second indicating lamp is lit up. That is, in the presence of a plurality of modes to be selected, indicating lamp-lighting circuits must be provided in the same number as the number of the modes to be indicated, which is very inconvenient.